The present invention relates to a device having a signal interpolation circuit for electrically interpolating (dividing) a signal having a predetermined phase and a displacement measuring apparatus comprising the device.
A conventional signal interpolation circuit shown in FIG. 1A is known. The circuit shown in FIG. 1A includes input terminals 1 and 2 for receiving sinusoidal wave signals of two different phase angles (0.degree. and 90.degree.), buffers 3 and 4, an inverting buffer 5 for inverting an input signal, conversion means 6 to 9 for converting a plurality of signals having different phase angles (0.degree., 45.degree., 90.degree., and 135.degree.) obtained by mixing output signals at a predetermined ratio through a circuit (resistor circuit) including resistors R.sub.1 to R.sub.4 into rectangular wave signals, pulse generating means 10 to 13 for generating pulse signals corresponding to signals from the resistor circuit on the basis of the rectangular wave signals from the conversion means, an OR gate 14 for obtaining an interpolation pulse on the basis of the outputs from the pulse generating means 10 to 13, and an output terminal 15. The resistors R.sub.1 to R.sub.4 have the same resistance.
FIG. 1B shows in detail an arrangement of the conversion means 6 to 9 and the pulse generating means 10 to 13. The circuit shown in FIG. 1B includes a comparator 61 which has a hysteresis given by resistors R.sub.A and R.sub.B (resistances r.sub.A and r.sub.B). The hysteresis is set to be positive/negative symmetrically about the center of a signal (zero-crossing point) by a bias voltage V.sub.B. The circuit also includes a buffer 101, an inverter 102, one-shot multivibrators 103 and 104, and an OR gate 105.
The operation will be described below. Sinusoidal wave signals respectively having phase angles of 0.degree. and 90.degree. are input to the input terminals 1 and 2. Thus, sinusoidal wave signals having phase angles of 0.degree., 90.degree., and 180.degree. are obtained from the outputs of the buffers 3 and 4 and the inverting buffer 5. Signals having phase angles 45.degree. and 135.degree. appear at nodes c and d in FIG. 1A since two signals are mixed by the resistors R.sub.1 to R.sub.4 The signals at these nodes have waveforms as shown in FIG. 1C.
The signals at nodes c to d are converted to rectangular wave signals by the conversion means 6 to 9, thus forming an interpolation pulse. This operation will be described below with reference to FIG. 1C. For example, the sinusoidal wave signal at the node a is converted to the rectangular wave signal by the comparator 61. At this time, if a peak-to-peak value of the output voltage of the comparator 61 is represented by V.sub.P, the hysteresis width described above is expressed by V.sub.P .times.{r.sub.A /(r.sub.A +r.sub.B)}. The rectangular wave signal passes through the buffer 101 and the inverter 102, and waveform signals e and f in FIG. 1C are obtained. At the leading edges of these signals e and f, the one-shot multivibrators 103 and 104 generate pulses, and a waveform signal g shown in FIG. 1C, i.e., a pulse signal corresponding to the above-mentioned phase angle is obtained as an output of the OR gate 105. The similar operation is performed in the conversion means 7 to 9 and the pulse generating means 11 to 13 having the same arrangement as described above, and a pulse train h in FIG. 1C is obtained as an output of the OR gate 14 at the final stage.
As a result, an interpolation pulse train corresponding to the phase angles 0.degree., 45.degree., 90.degree., and 135.degree. and their inverted phase angles 180.degree., 225.degree., 270.degree., and 315.degree. is obtained. Thus, a period of an input sinusoidal wave signal can be divided into eight sections.
However, in the conventional interpolation circuit, since the amplitudes of the sinusoidal wave signals (having phase angles of 45.degree. and 135.degree.) formed upon signal mixing by the resistors are smaller than those of the sinusoidal wave signals having the phase angles of 0.degree. and 90.degree. corresponding to original signals input to the terminals 1 and 2, the durations of rectangular wave signals generated through the comparators 6 of the conversion means 6 to 9 are varied. Therefore, an interpolation pulse interval also varies, and interpolation precision is degraded. In particular, when a large hysteresis width is set to prevent an erroneous operation caused by external noise or the like, interpolation precision is further degraded.